The present invention relates to a semiconductor integrated circuit which is used for a stabilizing power supply and a method for operating the same, and particularly, to a technology that is effective to reduce an increase of the number of external components, in a case that instability or abnormal oscillation of a stabilizing power supply due to an increase of an equivalent series resistance (ESR) of an electrolytic capacitor at the time of low temperature is prevented.
As well known, stabilized power supplies include a series regulator and a switching regulator.
The series regulator has an output control circuit which is connected in series between an input voltage and a load, and a voltage drop of the output control circuit is controlled by an error amplifier. The error amplifier compares a fractional voltage of the output voltage of the load with a reference voltage, controls the output control circuit in such a manner that the fractional voltage of the output voltage of the load is equal to the reference voltage, and thus, the output voltage of the load is stabilized. The series regulator is described in, for example, JP-A-2000-284843.
Switching regulators are divided into a type having a voltage step-down function and a type having a voltage boosting function. A switching regulator having a voltage step-down function is configured by a switching circuit having a first switching element and a second switching element, a low pass filter having a smoothing coil and a smoothing capacitor, an error amplifier, and a switching control circuit. An input voltage can be supplied to an end of the smoothing coil via the first switching element, a ground voltage can be supplied to an end of the smoothing coil via the second switching element, and the other end of the smoothing coil is connected to the ground voltage via a parallel connection of the smoothing capacitor and the load. During an ON period when the first switching element is controlled to have an ON state and the second switching element is controlled to have an OFF state by the switching control circuit, a coil current and the output voltage of the load increase, and during an OFF period when the first switching element is controlled to have an OFF state and the second switching element is controlled to have an ON state by the switching control circuit, the coil current and the output voltage of the load decrease. The error amplifier compares the fractional voltage of the output voltage of the load with the reference voltage, controls the switching control circuit in such a manner that the fractional voltage of the output voltage of the load is equal to the reference voltage, adjusts an ON period TON and an OFF period TOFF, and thus, the output voltage of the load is stabilized. The output voltage VOUT of the switching regulator having the voltage step-down function is represented by the following Formula (1) with an input voltage VIN, the ON period Ton, and the OFF period TOFF.VOUT=VIN·TON/(TON+TOFF)  Formula (1)
A switching regulator having a voltage boosting function is configured with a smoothing coil, a switching circuit having a first switching element and a second switching element, a smoothing capacitor, an error amplifier, and a switching control circuit. An input voltage is supplied to an end of the smoothing coil, the other end of the smoothing coil is connected to an end of the first switching element and an end of the second switching element, the other end of the first switching element is connected to a ground voltage via a parallel connection of the smoothing capacitor and a load, and the other end of the second switching element is connected to the ground voltage. During an ON period when the first switching element is controlled to have an OFF state and the second switching element is controlled to have an ON state by the switching control circuit, a coil current increases, and during an OFF period when the first switching element is controlled to have an ON state and the second switching element is controlled to have an OFF state by the switching control circuit, the coil current decreases. The error amplifier compares the fractional voltage of the output voltage of the load with the reference voltage, controls the switching control circuit in such a manner that the fractional voltage of the output voltage is equal to the reference voltage, adjusts an ON period TON and an OFF period TOFF, and thus, the output voltage of the load is stabilized. The output voltage VOUT of the switching regulator having the voltage boosting function is represented by the following Formula (2) with an input voltage VIN, the ON period Ton, and the OFF period TOFF.VOUT=(1+TON/TOFF)·VIN  Formula (2)
A switching regulator having the voltage step-down function and the voltage boosting function is described in, for example, JP-A-2007-151246.
In “‘Application to smoothing capacitor of switching power supply’ pp. 65-69. [accessed on 2013 Nov. 7]”, it is described that if a small equivalent series resistance (ESR) is selected so as to suppress an output ripple voltage for an output smoothing capacitor of a switching power supply, a phenomenon which is called an abnormal oscillation of the output voltage is generated. In “Application to smoothing capacitor of switching power supply′ pp. 65-69. [accessed on 2013 Nov. 7]”, it is described that the switching power supply has a negative feedback circuit for stabilizing the output voltage, and thus, if the output smoothing capacitor having a small equivalent series resistance (ESR) is used, the output smoothing capacitor becomes an ideal LC filter below a higher frequency band, and the phase is delayed by approximately 180 degrees, thereby oscillation easily occurs. Furthermore, in “‘Application to smoothing capacitor of switching power supply’ pp. 65-69. [accessed on 2013 Nov. 7]”, it is described that a feedback circuit of the error amplifier performs phase compensation and thus, the oscillation of the output voltage is prevented. A phase compensation circuit for preventing oscillation is configured with a series connection or a parallel connection of an input resistor and an input capacitor and is provided between an output terminal of a junction of a smoothing coil L of the switching power supply and an output smoothing capacitor Cout and an inverting input terminal of the error amplifier, or with a series connection or a parallel connection of a feedback resistor and a feedback capacitor and is provided between an inverting input terminal of the error amplifier and an output terminal of the error amplifier.
In “‘Application note surface mounted chopper type switching regulator IC SI-8000JD series’ pp. 1-29, Sanken Electric Co., Ltd., 2010 January [accessed on 2013 Nov. 7]”, in the same manner as in “‘Application to smoothing capacitor of switching power supply’ pp. 65-69. [accessed on 2013 Nov. 7]”, it is described that in a PWM control chopper type regulator, a phase delay of −180.degree. is generated by an LC filter configured with a smoothing coil and an output smoothing capacitor, a phase delay of −180.degree. is generated by a negative feedback loop, and a total phase gap reaches 360.degree., and thereby abnormal oscillation is generated. Furthermore, in “‘Application note surface mounted chopper type switching regulator IC SI-8000JD series’ pp. 1-29, Sanken Electric Co., Ltd., 2010 January [accessed on 2013 Nov. 7]”, it is described that the phase delay of the LC filter is smaller than 180.degree. by an influence of the equivalent series resistance (ESR) of the output smoothing capacitor, and a phase margin of the regulator can be secured by a phase compensation effect of the equivalent series resistance (ESR). In addition, in “‘Application note surface mounted chopper type switching regulator IC SI-8000JD series’ pp. 1-29, Sanken Electric Co., Ltd., 2010 January [accessed on 2013 Nov. 7]”, it is described that if the equivalent series resistance (ESR) is large, the phase delay of the LC filter is small, but a phase margin for a gain increase is decreased due to a decrease of the attenuation rate of the LC filter, and an abnormal oscillation can occur due to an increase of an output ripple voltage. Furthermore, in “‘Application note surface mounted chopper type switching regulator IC SI-80001D series’ pp. 1-29, Sanken Electric Co., Ltd., 2010, January [accessed on 2013 Nov. 7]”, it is described that if a capacitor having an extremely small equivalent series resistance (ESR), such as a tantalum capacitor or a laminated ceramic capacitor, is used for the LC filter, the phase delay of the filter is large, and thus, from a viewpoint of phase margin security, using an electrolytic capacitor for an output filter is appropriate.
In “‘Notes on use of operational amplifier with small phase and gain margin’ pp. 1-8, [accessed on 2013 Nov. 7]”, it is described that if an input signal is supplied to a non-inverting input terminal of an operational amplifier, and a voltage follower circuit is configured by connecting an inverting input terminal of the operational amplifier to an output terminal of the operational amplifier, a phase delay having a peak with a gain of more than 0 dB at a frequency of approximately 100 MHz to 200 MHz reaches −180.degree., and the operation is unstable. Furthermore, in “‘Notes on use of operational amplifier with small phase and gain margin’ pp. 1-8, [accessed on 2013 Nov. 7]”, it is described that a series connection of an input resistor and an input capacitor is connected between a non-inverting input terminal and an inverting input terminal of the operational amplifier, thereby compensating for the phase, and thus, there is no peak with a gain of more than 0 dB, and oscillation can be prevented. In addition, in “‘Notes on use of operational amplifier with small phase and gain margin’ pp. 1-8, [accessed on 2013 Nov. 7]”, it is described that a series connection of a compensation resistor and a compensation capacitor is connected between an output terminal of the operational amplifier and a ground voltage, thereby compensating for the phase, and thus, there is no peak with a gain of more than 0 dB, and oscillation can be prevented.
In JP-A-2002-136123, there are described problems of increase in the output ripple voltage because an equivalent series resistance (ESR) of an electrolytic capacitor connected to a secondary coil of a transformer of the switching power supply increases at a low temperature, and stability of a control system being affected because the equivalent series resistance (ESR) decreases at a high temperature. In order to solve these problems, in JP-A-2002-136123, in consideration of temperature characteristics of the electrolytic capacitor, the deeper negative feedback is set so as to be used stably in a wide temperature range. Thus, in JP-A-2002-136123, an output voltage control circuit which is connected to the secondary coil of the transformer of the switching power supply includes a phase compensation circuit with a temperature compensation function, and specifically in the phase compensation circuit with a temperature compensation function, a series connection of a thermistor, a capacitor, and a resistor, which are temperature elements, are connected between the secondary coil of the transformer and a reference terminal of the shunt regulator. At a low temperature, the equivalent series resistance (ESR) of the electrolytic capacitor increases, and thereby the output ripple voltage increases, but the resistance value of the thermistor of the phase compensation circuit with a temperature compensation function is increased at the low temperature, and thus an amount of superimposition of output ripple components with the reference terminal of the shunt regulator is reduced, and thereby a phase leading signal of the shunt regulator becomes small. At a high temperature, the equivalent series resistance (ESR) of the electrolytic capacitor decreases, the resistance value of the thermistor of the phase compensation circuit with a temperature compensation function is decreased, and thus, an amount of superimposition of the output ripple components with the reference terminal of the shunt regulator is increased, and thereby a phase leading signal of the shunt regulator is maintained constant. Meanwhile, in the switching power supply, an amount of received feedback light of a light receiving element of a photocoupler changes according to an amount of emitted light of a light emitting element of the photocoupler according to the output voltage of the secondary coil of the transformer. An IC for control which is connected to a primary coil of the transformer controls ON and OFF duty of two switching elements which are connected to the primary coil, according to an amount of the received feedback light, and thus, the output voltage of the secondary coil of the transformer is maintained constant. Furthermore, a phase compensation circuit on a primary side which is configured by a series connection of a resistor and a capacitor is connected in parallel between both ends of the light-receiving element of the photocoupler.
In JP-A-2002-044938, there are described problems of destabilizing of an output voltage caused by an increase of an output ripple voltage because an equivalent series resistance (ESR) of an output smoothing capacitor in an LC filter which is configured by a smoothing coil and the output smoothing capacitor increases at a low temperature, and of generating unintended activation of a switching power supply device, in a voltage step-down DC-DC converter as the switching power supply device. In order to solve these problems, it is described that a ripple voltage detection circuit, a comparator, a switching circuit, and a ripple voltage suppressing circuit are connected to an inverting input terminal of a differential amplifier as an error amplifier, in a control unit that controls ON and OFF duty of a switching element which drives the LC filter in JP-A-2002-044938. The ripple voltage detection circuit detects a ripple voltage included in the output voltage of the switching power supply device, and supplies the detected value to the comparator. If the ripple voltage included in the output voltage of the switching power supply device is increased more than a comparison DC voltage, the comparator supplies a switching signal to the switching circuit. If the ripple voltage included in the output voltage of the switching power supply device is decreased more than the comparison DC voltage, the ripple voltage suppressing circuit which is a low pass filter is not connected to an inverting input terminal of the differential amplifier by the switching circuit. In contrast, if the ripple voltage included in the output voltage of the switching power supply device is increased more than the comparison DC voltage, the ripple voltage suppressing circuit which is a low pass filter is connected to an inverting input terminal of the differential amplifier by the switching circuit. As a result, it is described in JP-A-2002-044938 that a signal with higher frequency components than those of the ripple voltage included in the output voltage of the switching power supply device is blocked, a signal with only frequency components which have low frequency compared to the frequency components of the ripple voltage included in the output voltage of the switching power supply device is supplied to the inverting input terminal of the differential amplifier, and thus, a feedback system becomes stable. Thus, it is described in JP-A-2002-044938 that the ripple voltage suppressing circuit makes a gain of the differential amplifier effectively small, and thus, even if the equivalent series resistance (ESR) of the capacitor increases at a low temperature and thereby the ripple voltage increases, the feedback system can be stabilized.